Which venting option is better?

I want to move a toilet to the other side of a small second floor bathroom. It would sit next to a new shower with a new wall separating the two. The new 2â€ shower drain would connect into the new 3â€ toilet drain, which would then run across the room about 6 feet to the existing soil/vent stack. The photos below show several options for the design of the drains and vents.

I canâ€™t run any vents up the wall behind the toilet and shower because of various obstacles, but I can run vents up the new wall between them (it will be about 8 inches wide and 32 inches deep).

The floor joists run parallel to the wall behind the toilet (perpendicular to the new drain as it runs across the room to the existing stack). The challenge is to find a drain/vent configuration that mostly fits between adjoining joists so as to comply with the notching and drilling restrictions that apply to the joists. I can replace all the joists with a variety of I-joists and oversized lumber joists (2 x 12s), spaced at irregular intervals as needed, but a number of things limit where the new joists could be located, e.g., light canisters in the suspended ceiling in the room below and various obstructions along the top plates on which the joists rest. The joist span is only 9 feet, and especially with oversized joists I see no issue as to meeting code minimum (apart from notches and holes), but I want the floor to be as sturdy as possible to avoid deflection cracks in the tile.

Iâ€™ve toyed around with several configurations shown in the photos, trying to come up with a design that would comply with code (IRC and Illinois Plumbing Code), function optimally, and allow an acceptable plan for locating and drilling the joists. The mock-ups show the basic designs, using pipe scraps that only approximate the actual lengths needed. Since the bathroom is on the second floor, I assume that no cleanouts are required.

Iâ€™d like to minimize the bends in the toilet drain. The centerline of the separating wall would be offset about 8 inches towards the toilet from the line directly across from the stack. The drain connection at the stack is low enough to allow the proper pitch in the drain lines, but it leaves only about 4 inches between the pipe and the subfloor to play around with angled vent lines. The width and depth of the separating wall give me some additional leeway as to where the vents come up through the floor.

Photo A shows a vent arrangement that I think fully complies with the plumbing requirements (spot any errors?), but it is very problematic from a joist perspective. The toilet vent is 2â€ and the shower vent is 1.5â€, and their inverts are above the drain centerlines. The two vents would join near the ceiling.

Big question: Do I actually need a second vent for the toilet (the existing cast iron stack continues upwards as a vent)? The developed length of the toilet drain would be more than 6 feet. The IRC seems to say that no vent would be necessary in addition to what exists at the stack. The Illinois Code seems to say that I would need a vent within 6 feet of the toilet (Sections 890.1470 and 1490). The issue is very confusing.

The other photos show options that are more â€œjoist friendlyâ€ by assuming either that no separate vent is needed for the toilet or that a 2â€ vent off the shower drain would satisfy what is needed for both fixtures.

Photo B is simply a 2â€ vent off the shower drain upstream of where it connects to the toilet drain. Photos C and D show options that might function like a wet vent (â€œverticalâ€ at 45 degrees), the primary difference being whether the branch of the 2 inch wye is used for the vent or for the shower drain.

Thanks for taking a look at this—I tend to favor D, too, but I’m not sure of the reasons why it provides adequate venting for the toilet. In particular—

1) Does the toilet actually not need a nearby vent (given the vent back at the stack)? If not, could the vent in photos B, C, and D just be 1.5 inches? The smaller size would make routing the vent line a bit easier.

2) If I do need a 2 inch vent within 6 feet of the toilet (as the Illinois Code seems to say—but I may be misreading that), is that satisfied by a vent coming off the shower drain line, even though the path of the vent for the toilet would be essentially be through a flat wye? For example, in option B if you eliminated the shower and replaced the 2 inch wye with an elbow, the end result would seem to be just a toilet vent line that joins the drain line horizontally (apparently, a no-no).

3) Which then suggests that maybe the shower drain line changes the whole situation by providing a flushing action to clean out the horizontal portion of the vent line (my understanding is that vents must come off the drain “vertically” so that any water that backs up into the vent line can adequately drain, as well as the vent invert not disrupting the smooth flow of water in the bottom half of the drain pipe). Is this covered by the concept of a “horizontal wet vent”?

4) Is the key actually that Options C and D provide a “vertical wet vent” as long as the wye in the toilet drain line is rotated so that the branch comes off at a 45 degree angle?

I’m wondering what latitude I have in positioning my wye fittings. The concept of a “vertical” vent (45+ degrees) seems to be a bit tricky when applied to connecting fittings. The branch of a wye will run at a true 45 degree angle only if the branch comes off at the very top. Nonetheless, it seems to be common practice to allow you to rotate the wye no more than 45 degrees from the top—even though the effective slope of the branch would then be shallower. This seems to be allowed if the branch then immediately goes into some sort of elbow so that the vent pipe itself is “vertical.” Is that how compliance works in the real world? Also, the notion of a 45 degree connection by rotating a wye seems to be just an approximation. I understand the true requirement to be that the invert of the connecting branch should be above the centerline of the drain, which would not be true for a standard wye that has been rotated 45 degrees. A reducing wye would allow a bit more of an angle than a standard wye because the smaller diameter of the branch means that the invert would be a bit higher than otherwise. True? Should I be concerned about any of these details?

1. In most codes, the minimum vent for a toilet is 2", but I believe the IPC allows 1 1/2" because they allow anything that its members want.
2. Do not try to equate a vent without the shower with what you have. Once you do that the dynamics change completely.
3.Yes.
4. B, C, adn D are ALL wet vents. The only differences are how the piping is arranged and that is a function of where the wall is that the vent will rise through. If the vent can go "any where", then put on a blindfold and throw a dart at your drawings and use the one it hits.
5. Once the vent is connected it makes no difference whether the drain line runs 6' or 60'.